MARTINEZ -- A Concord man awaiting trial for allegedly killing a 21-year-old woman for the deed of a foreclosed house has changed his plea from not guilty to not guilty by reason of insanity.

A Contra Costa County judge last week appointed two doctors to determine whether they think 24-year-old Donald Paul Waldecker was legally insane when he killed Shelby Barnes at her Concord home on Oct. 24, 2011, said deputy district attorney Jason Peck. The doctors' reports are due to be returned to the court June 18.

Waldecker and co-defendants Randi Len Dees, 21, of Richmond, and Johnathan Ismael Harriel, 18, of Concord, were each ordered to trial on murder charges at a preliminary hearing last year. Harriel was 17 when the slaying occurred but is being tried as an adult. Their trial dates have not been scheduled.

Waldecker was among several young people whom Barnes, a Benicia native, allowed to stay with her at a Glenside Court residence that had been owned by her father before it was seized by a bank. Detectives have testified that Waldecker thought that if he could get the deed from Barnes, the property would belong to him, despite people telling him that there was no deed in Barnes' possession.

Police said Waldecker confessed to kicking Barnes down a flight of basement stairs, bashing in her head with a wooden table leg and slitting her throat. His co-defendants are being prosecuted under an aider-and-abettor theory.

Waldecker's attorney could not be immediately reached for comment Thursday.